Even if it was to unlock a new level in a game, or to remove a tooth? :)
More seriously though seems like i didn't manage to express what i wanted to say in my comment.
My point was that people are afraid of torture, and many other things (like roller coasters) because there's a large chance to die in the end. And without that chance the whole thing is just some neurons being active, and is not a big deal.
Your point about needing to erase all memory of the experience to not get mental illness is very interesting. I'd expect the brain to be more stable, but we don't know enough about the brain to say for sure.
I actually didn't express that very well. I kind of meant that if you were to erase the mental illness, you've effectively erased the whole experience. Horrible torture isn't something you just leave behind you the day after. The influence it has is almost necessarily mental illness in some form or fashion.
If anyone did go through horrible torture voluntarily, I would expect they would do it to know what it was like. And mental illness is part of what it's like.
There are people afraid of bugs, dogs, cats, birds, balls etc... There are also people who experience anxiety for unknown reasons due to anxiety disorder. Moreover, there is no, quoting you, "large chance to die in the end" in roller coasters. I don't think any part of your argument makes sense, sorry.
The point of tooth removal in dentistry is to end pain, not to cause it.
I certainly wouldn't do it for "a game." I don't think I would do it for any amount of money or other benefit, either. What you're proposing is (seemingly) no benefit beyond the experience of being tortured--which coincidentally you are going you erase when you delete the mental illness.
Anyway, you're wrong, nobody would sign up for that. People who are afraid of roller coasters don't ride them. Everyone's afraid of torture.